This invention relates to brushless motors utilizing Hall elements as the sensor for detecting the position of the rotor, and more particularly to the mounting structure of the Hall elements therein.
Brushless motors utilizing Hall elements as the sensor for detecting the rotor position are now widely used in the field where precise control of the rotation of the motor is essential. In such motors, the rotational position of the rotor is detected by the Hall elements, and, in response thereto, the directions of the currents supplied to the stator coils are switched over in turn by power transistors so as to produce a rotating magnetic field and to obtain the torque of the rotor. However, the outputs of the Hall elements, which utilize the Hall effect to detect the magnetic field generated by a magnet fixed on the rotor shaft, are dependent on their attitude. Thus, it is important that the detector surface of the Hall elements are precisely positioned with respect to the magnet fixed on the rotor shaft.
Japanese Patent Publication No. 63-1020 discloses a mounting structure of Hall elements in brushless motors whereby a double-sided printed circuit board is utilized: the printed circuit board comprises notches formed on the inner surface of the annular portion thereof surrounding an axial end of the rotor, and the Hall elements are fitted into these notches so as to bridge the printed circuit patterns formed on both surfaces of the circuit board. This method of mounting, however, requires a double-sided printed circuit board and is rather complicated. FIGS. 1 through 3, on the other hand, show a simpler mounting structure in which a one-sided printed circuit board is utilzed for the mounting of the Hall elements:
FIG. 1 shows the overall structure of a frameless type brushless motor in which the Hall elements are mounted by means of a one-sided printed circuit board. The stationary part of the motor includes: an annular stator core 1, stator coils 1a wound around the stator core 1, and cup-shaped front and rear brackets 2a and 2b secured to the stator core 1 to form the housing structure of the motor. On the other hand, the rotor 3 of the motor comprises: a rotor shaft 4 rotatably supported by a pair of bearings 5a and 5b sucured to the front and rear brackets 2a and 2b, respectively; and a rotor core 6a and a main magnet 6b coaxially secured to the rotor shaft 4. Thus, the rotor 3 is free to rotate within the stator core 1. Further, to the rear side of the rotor 3 (to the right in the figure), an annular sub-magnet 7 is secured to the rotor shaft 4 to generate the magnetic field for the determination of the position of the rotor 3.
The mounting structure of the Hall elements is best shown in FIG. 2 which shows the cross section of the motor along the line A--A of FIG. 1. The Hall elements 8 (three in number in the fugure) are secured via respective holders 9 of a non-magnetic electrically insulating material to a semi-circular annular one-sided printed circuit board 10, so as to oppose the outer side surface of the submagnet 7 across a small radial gap. The Hall elements 8, which are, as is well known, molded into packages of epoxy resin, etc., are circumferentially spaced from each other by 60 degrees, and detect the variation of the magnetic field generated by the sub-magnet 7 to determine the position of the rotor 3. The semi-circular printed circuit board 10 having a printed circuit formed on the rear surface (at the right side in FIG. 1) is secured to the bottom surface of the rear bracket 2b via a plurality of fixing screws 11. As best shown in FIG. 3, each Hall element 8 is partially inserted into a holder 9 at the rear end thereof, to be supported by the holder 9 in the state of a cantilever, each Hall element 8 and holder 9 being fixed to each other and to the printed circuit board 10 by means of an adhesive 12; in addition, the four leads 8a of each Hall element 8 are electrically coupled via solder 13 to the electrically conductive pattern 10a formed on the rear surface of the printed circuit board 10.
The above mounting structure of the Hall elements 8 in brushless motors has the following disadvantage. Namely, the Hall elements 8 are each supported by the holder 9 in the state of a cantilever; thus, when the Hall element 8 are subjected to vibrations, especially to those in the direction Y shown in FIG. 3, the leads 8a and the solder 13 may suffer an electrical disconnection due to the fatigue failure thereof, which results in the disablement of the motor. Further disadvantage of the above mounting structure is that the application of the adhesive 12 requires an expert skill and is low in the operation efficiency.